Honda Rafaga Speedometer

My '95 Honda Rafaga's speedometer normally operates properly, however sometimes when starting the car it will stick at 20kph and stay there regardless of my speed or whether going forward or reverse for a short time before operating properly. This is also without regard to whether or not the engine is hot or cold. I have also seen it lag and jump at times when both accelerating and decelerating usually at 40-60kph. I have not been able to raise the car to inspect the cable or gear linking to the speedometer but I have been doing some research and read that it may possibly be the speedometer itself. I would like to know what kind of diagnostics I can do to figure out exactly what is causing this problem so that I can repair it.